1. Field of the Invention
The present invention relates to an inter-wavelength light power deviation monitoring method for monitoring, by a simple procedure with good precision, the wavelength characteristics of light power of wavelength-division multiplexed (WDM) signal light utilized in various optical communications, and to an optical equalizer and an optical amplifier for controlling the equalizing treatment for the wavelength characteristics of light power of WDM signal light by such a monitoring method.
2. Related Art
In optical communications, research and development have been vigorously done on a wavelength-division multiplexing (WDM) transmission system to increase a communication capacity by transmitting light having a plurality of wavelengths through a single optical fiber. Such a WDM transmission system is expected to become the means for realizing a future multi-media based world, because of such advantages that the system can utilize an existing optical fiber, leading to a lower introduction cost, and that the transmission path of the system can be used in a bit-rate free manner by adopting an optical amplifier having a wider amplification band, leading to easiness of future upgrade.
Generally, in a WDM optical communication system adopting optical amplifiers, in order to obtain predetermined transmission characteristics, it is required to transmit light by suppressing the deviation (tilt) of light power between channels to a few dB or less at each amplification repeating stage. This is because the upper limit of transmittable light power is restricted by the waveform degradation due to a nonlinear effect of an optical transmission path, and the lower limit is restricted by the degradation of a received S/N ratio.
It is known that wavelength characteristics are caused in the loss of an optical fiber transmission path such as due to stimulated Raman scattering or Rayleigh scattering as one of nonlinear effects. Particularly, the loss wavelength characteristics due to stimulated Raman scattering are caused in different magnitudes depending on various transmission conditions such as a length of an optical transmission path, the number of channels (i.e., the number of wavelengths of signal light), channel intervals (wavelength intervals of signal light), and a light power level. Typically, such as the number of channels and the wavelengths to be used in a WDM optical transmission system can be suitably set by a system user, so that the values of them are not always fixed. For example, in a lightwave network, such as the number of signal light and wavelength positions to be input into an optical ADM (Optical Add and Drop Multiplexer) device change dynamically, so that loss wavelength characteristics having different magnitudes are caused in optical transmission paths, leading to a serious problem of affection on transmission characteristics.
With respect further to optical amplifiers and dispersion compensators constituting various optical transmission systems, it is known that wavelength characteristics are included in gains and losses, and it is a problem that such wavelength characteristics in gains and losses cause light power deviations among respective channels, largely affecting on the transmission characteristics.
Thus, it is required to take a countermeasure to monitor light power deviations among respective channels caused in a WDM optical communication system, to thereby reduce such deviations. To this end, the present inventors have proposed such a technique to apply an active gain equalizer to a WDM optical communication system so as to control the characteristics of the gain equalizer such that monitored results of the light power deviations among respective channels are reduced (see Japanese Patent Application Nos. 11-54374 and 11-115971).
Conventional monitoring methods for monitoring the light power deviations among respective channels such as applied to the aforementioned patent applications include a method for calculating the light power deviations among channels based on amplified spontaneous emission (ASE) light generated by optical amplifiers (see Japanese Patent Application No. 11-054374). According to such a method, the spontaneous emission light power near signal light bands are monitored so that the dispersions of light power among respective channels can be monitored independently of the number of input signals or the variation of channels. Further, there is also known a method to directly measure the light power of respective channels such as by a general optical spectrum analyzer to thereby calculate light power deviations among respective channels.
However, those monitoring methods utilizing spontaneous emission light among the conventional methods for monitoring the inter-wavelength light power deviations are ones to indirectly monitor spontaneous emission light rather than directly monitoring signal light itself, leading to the possibility of monitoring error. Also, in those monitoring methods for directly measuring the respective signal light power by using such as an optical spectrum analyzer, since the respective signal light power should be accurately measured even when signal light are input into signal light bands in the densest manner, the measurement of signal light power should be conducted by using such as the high grade optical spectrum analyzer having a superior wavelength resolution, problematically resulting in a large-sized apparatus with higher cost in an apparatus adopting such a monitoring method.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a monitoring method capable of directly measuring the signal light power to thereby detect with high precision the light power deviations among channels without requiring a higher wavelength resolution, and to provide an optical equalizer and an optical amplifier adopting such a monitoring method to thereby attain downsizing and cost reduction.
To achieve the above object, the present invention provides a method for monitoring inter-wavelength light power deviations, comprising the steps of: obtaining channel information concerning WDM signal light including a plurality of wavelength-division multiplexed channel light having wavelengths different from one another, setting, based on the channel information, a plurality of measuring wavelength regions each including at least one distinctive channel light; measuring channel light power of the WDM signal light, for only the respective measuring wavelength regions, and obtaining light power ratio for the respective measuring wavelength regions making use of measured values of said light power, to thereby judge the inter-wavelength light power deviations of the WDM signal light.
According to such a monitoring method: there are set at least two measuring wavelength regions suitable for light power measurement based on the channel information concerning such as the wavelength positions and transmission rates of respective channel light included in the WDM signal light, channel light power for only the respective measuring wavelength regions are measured, and the inter-wavelength light power deviations are judged from the light power ratio for the respective measuring wavelength regions, based on the measured results. In this way, it becomes possible to monitor the wavelength characteristics of WDM signal light power with higher precision while directly measuring the signal light power at a relatively low resolution of wavelength.
Concerning the plurality of measuring wavelength regions each including at least one distinctive channel light, it is possible to arrange such that each of the measuring wavelength regions includes a singular distinctive channel light, and these distinctive channel light are different from one another, or, each of the measuring wavelength regions includes a plurality of channel light, and at least one of the plurality of channel light is different from the remaining measuring wavelength regions. Alternatively, the plurality of measuring wavelength regions are provided by combining the two situations described just above.
Concerning the above monitoring method, the step of setting the measuring wavelength regions may set at least a first measuring wavelength region including the shortest wavelength channel light and a second measuring wavelength region including the longest wavelength channel light, among the input channels. It is desirable to set each of the measuring wavelength regions such that the inclusion ratio of noise light becomes minimum. In this way, it becomes possible to improve the measuring precision of light power deviations of the respective channel light.
Concretely, the step of obtaining the channel information may obtain the channel information including of the wavelength positions and the transmission rates of respective channel light in the WDM signal light, based on monitoring signal light transmitted together with the WDM signal light.
The present invention further provides an optical equalizer for equalizing wavelength characteristics of the light power of WDM signal light including a plurality of wavelength-division multiplexed channel light having wavelengths different from one another, which comprises variable light-equalizing means having a variable light equalizing characteristic, to which is input the WDM signal light, channel information obtaining means for obtaining channel information concerning the WDM signal light; wavelength region setting means for setting, based on the channel information obtained by the channel information obtaining means, a plurality of measuring wavelength regions each including at least one distinctive channel light, light power measuring means for measuring channel light power of the WDM signal light passed through the variable light-equalizing means, for only the respective measuring wavelength regions set by the wavelength region setting means, and controlling means for obtaining, based on the channel information obtained by the channel information obtaining means, target values of light power ratio for the respective measuring wavelength regions, to flatten the light power wavelength characteristics of the WDM signal light and for obtaining measured values of light power ratio for the respective measuring wavelength regions, making use of the measured result of the light power measuring means, to control the light equalizing characteristic of the variable light-equalizing means so that the measured values become equal to the target values, respectively.
Further, one aspect of an optical amplifier according to the present invention incorporating optical amplifying means for collectively amplifying WDM signal light including a plurality of wavelength-division multiplexed channel light having wavelengths different from one another, comprises variable gain equalizing means having a variable gain equalizing characteristic, to which is input the WDM signal light; channel information obtaining means for obtaining channel information concerning the WDM signal light, wavelength region setting means for setting, based on the channel information obtained by the channel information obtaining means, a plurality of measuring wavelength regions each including at least one distinctive channel light, light power measuring means for measuring channel light power of the WDM signal light passed through the variable gain equalizing means, for only the respective measuring wavelength regions set by the wavelength region setting means, and controlling means for obtaining, based on the channel information obtained by the channel information obtaining means, target values of light power ratio for the respective measuring wavelength regions, to flatten the light power wavelength characteristics of the WDM signal light and for obtaining measured values of light power ratio for the respective measuring wavelength regions, making use of the measured result of the light power measuring means, to control the gain equalizing characteristic of the variable gain equalizing means so that the measured values become equal to the target values, respectively.
In the optical equalizer or optical amplifier having the aforementioned constitution, the aforementioned monitoring method according to the present invention is realized by the operations of the channel information obtaining means, wavelength region setting means, light power measuring means and controlling means, and the equalizing characteristic of the variable light-equalizing means or the variable gain equalizing means is feedback controlled corresponding to the monitored result. This enables the provision of a downsized and economical optical equalizer or optical amplifier capable of assuredly conducting the equalizing treatment of light power wavelength characteristics of WDM signal light.
As a concrete constitution of the aforementioned optical equalizer or optical amplifier, the light power measuring means may include a diffraction grating for receiving a part of the WDM signal light passed through the variable light-equalizing means, to thereby generate diffracted light, and a light receiving part which has a plurality of photodetectors arranged corresponding to the arriving positions of respective channel light diffracted by the diffraction grating; and which selects the measured results of light power at the photodetectors corresponding to the respective measuring wavelength regions set by the wavelength region setting means, to pass the thus selected measured results to the controlling means. In addition, the light receiving part can include two or more photodetectors the number of which is equal to or less than the maximum number of channels included in the WDM signal light.
According to the light power measuring means having such a constitution, the channel light power are measured for the corresponding photodetectors, respectively, making use of the light diffraction by the diffraction grating, and the measured results of the photodetectors corresponding to the respective measuring wavelength regions are selectively transmitted to the controlling means.
Alternatively, as another concrete constitution of the light power measuring means, it may include: a variable optical filter, which is input with a part of the WDM signal light passed through the variable light-equalizing means, and which has a central wavelength of a transmission band to be shifted in a timewise manner corresponding to the respective measuring wavelength regions set by the wavelength region setting means; a light receiving part for measuring power of channel light passed through the variable optical filter, and a selecting part for selecting, from measured results at the light receiving part, data corresponding to the respective measuring wavelength regions set by the wavelength region setting means, to transmit the selected data to the controlling means.
According to the light power measuring means having such a constitution, the power of the respective channel light are measured by the light receiving part corresponding to the timewise shift of the transmission band of the variable optical filter, and the measured results of the photodetector, at the time the transmission band of the variable optical filter corresponds to each of the measuring wavelength regions, are selectively transmitted to the controlling means.
Further, as another aspect of the optical amplifier according to the present invention, the optical amplifier incorporating demultiplexing means for demultiplexing WDM signal light including a plurality of wavelength-division multiplexed channel light having wavelengths different from one another into at least two amplification-wavelength bands, at least two optical amplifying means for amplifying optical signals of the respective amplification-wavelength bands, which are demultiplexed by the demultiplexing means, and multiplexing means for multiplexing respective optical signals amplified by the respective optical amplifying means, comprises variable gain equalizing means, which has a variable gain equalizing characteristic and which is provided at the preceding stage of an input side of the demultiplexing means; channel information obtaining means for obtaining channel information concerning the WDM signal light, input power measuring means for receiving a part of optical signals of the respective amplification-wavelength bands to be input into the respective optical amplifying means, to measure the total input-light power into the respective optical amplifying means, and controlling means for obtaining, based on the channel information obtained by the channel information obtaining means, target values of total input-power ratio for respective optical amplifying means, to flatten the light power wavelength characteristics of the WDM signal light and for obtaining measured values of total input light power ratios for the respective optical amplifying means, making use of the measured result of the input power measuring means to control the gain equalizing characteristic of the variable gain equalizing means so that the measured values become equal to the target values, respectively.
According to the optical amplifier having such a constitution, the WDM signal light having a wider signal light band is sent via the variable gain equalizing means to the demultiplexing means and demultiplexed into a plurality of amplification-wavelength bands. The demultiplexed optical signals of the respective amplification-wavelength bands are amplified by the corresponding optical amplifying means, and then multiplexed by the multiplexing means. At this time, a part of the optical signals to be input into the respective optical amplifying means are sent to the input power measuring means, the total input-light power are monitored, and the gain equalizing characteristic of the variable gain equalizing means provided at the input stage is feedback controlled by the controlling means corresponding to the monitored results. In this case, the respective amplification-wavelength bands correspond to the measuring wavelength regions in the aforementioned monitoring method, and this monitoring method according to the present invention is realized by the operations of the channel information obtaining means, input power measuring means and controlling means, respectively. In this way, the tilt of light input into the respective optical amplifying means having different amplification-wavelength bands from each other can be assuredly compensated by utilizing the simple monitoring method. This constitution has two important points. The first point is that the tilt is compensated at the input side of the optical amplifying means to thereby relieve the noise figure of a channel wherein the transmission light power has been deteriorated, so that not only the tilt of the optical amplifier is compensated but also the wavelength characteristics of noise figures are suppressed, resulting in an advantage to the optical SIN ratio. The second point is that a common optical amplifier is already applied with a function for monitoring an input power so that a part of input light is branched and the thus branched light power is received by a photodiode. The monitoring method of the present invention utilizes such a constitution, so that no optical parts are required to be newly inserted.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.